The structures of cytochrome b562 from E. coli and of flavocytochrome b2 (L-(plus)-lactate dehydrogenase) from bakers yeast will be studied by x-ray diffraction methods. The major goals will be to understand the mechanism of electron transport at the molecular level. Cytochrome b562 appears to be different from other cytochromes of known structure, resembling myoglobin in sequence rather than cytochrome b5 or cytochrome c. It crystallizes in a triclinic cell with two molecules per asymmetric unit. Flavocytochrome b2 contains both a flavin and a heme prosthetic group. It catalyzes the dehydrogenation of L-lactate, transferring elctrons to cytochrome c in the mitochondria. It contains trypsin resistant heme peptide core which resembles microsomal cytochrome b5 in sequence and spectrum. Cytochrome b2 crystallizes in a trigonal cell with one tetramer of 230,000 Daltons per asymmetric unit. Large crystals have been grown which diffract to at least 3.0 angstroms resolution. The isomorphous replacement method of structure investigation will be coupled with the molecular replacement method to solve these structures. The structure-function relationships and comparison with other proteins, as well as electron density fitting, will be carried out on the MMS-X molecular graphics system.